Microsoft has flipped the switch and disabled access to the Windows Phone Marketplace on devices that have yet to update to Windows Phone 7.5 “Mango.” This change should only affect a very small number of users, since the software giant’s data shows that the vast majority of Windows Phones are already running the Mango update.

Released last fall, Windows Phone 7.5 “Mango” added more than 500 features to Microsoft’s smartphone OS, including a number of important improvements to the Marketplace itself. This is a must-have update, and it has been available on all devices on all carriers for some time now.

If you haven’t yet updated to Mango yet—and really, why wouldn’t you?—you’ll encounter an error whenever you attempt to “download, buy, update, or review apps in the Marketplace.” You’ll still be able to browse the Marketplace, but nothing else will work. This applies to both the Marketplace on the device itself and the one on Windows Phone.com. To continue using the Marketplace, simply connected your device to your computer, fire up the Zune software client or the Windows Phone Connector for Mac, and update the phone’s OS.